You have a real unique style, but I'm going to pass down some knowledge someone very wise once told me:
a lot of the designs that get printed at threadless are things with either strong ideas, or amazing art. your drawings are nice, but there's no real unifying concept or idea in them that makes them work as a shirt. i.e.; the fish are drawn really well, but they're just fish on a tee.
a similar design that got printed...
see how it just has an underlying concept that takes it to another level?
yeah, I hope my advice helps you. keep up the good work though, these are competent first subs!